So far, there has been a type of the cap for a penholder, preventing evaporation of ink at the pointed end of a pen being substantially air-tight. To secure ventilation to the direction of axis in case the cap is accidentally swallowed, it sets the inner cylindrical body in the outer cylindrical body and secures ventilation to the direction of axis through the space between inner and outer cylindrical body and also it covers and makes substantially air-tight a pointed end of a writing instrument by the inner cylindrical body (Japanese utility model open number 1-139595, Japanese utility model open number 1-1693 85).
There has also been a cap providing a vertical groove placed side by side on the outside circumference which is connected to the inside of the cap thus securing ventilation to the direction of axis through the groove in case the cap, is accidentally swallowed.
However, the formation of a number of above-said prior art caps specially in one body with inner and outer cylindrical body separated would complicate the production process, and composition parts would increases in number, and consequently it cost a great deal even if safety is maintained. Providing the air passage on outside circumference to the direction of axis from the closed end edge through inside the cap body up to the open end edge causes restriction to the use of a cap for the reason of production and design. Moreover, the rib provided on the circumference causes incomplete formation of the substantially air-tight area inside the cap through the process of formation thus it would be defective in providing air tightness.
Some defects of the prior art caps are improved by this device. It is formed in one body, is easy to form, lowers the cost, and improves a substantially air-tight condition while preventing suffocation in case the cap is swallowed.